D19 — Agent contract: AGENTS.md generated by Init

kb.Init creates AGENTS.md with soft guidance on style, naming, workflow, when to lint. Added to reservedNames in okf (does not appear in WalkConcepts).

Superseded by D62: kb.Init no longer generates AGENTS.md. The filename remains in reservedNames/IsReserved (backward compatibility: KBs that still have it from a previous Init keep working, the file is simply ignored by the server).